Cadence detection based on inertial harmonics

ABSTRACT

The method and apparatus disclosed herein determine a user cadence from the output of an inertial sensor mounted to or proximate the user&#39;s body. In general, the disclosed cadence measurement system determines the user cadence based on frequency measurements acquired from an inertial signal output by the inertial sensor. More particularly, a cadence measurement system determines a user cadence from an inertial signal generated by an inertial sensor, where the inertial signal comprises one or more frequency components. The cadence measurement system determines a peak frequency of the inertial signal, where the peak frequency corresponds to the frequency component of the inertial signal having the largest amplitude. After applying the peak frequency to one or more frequency threshold comparisons, the cadence measurement system determines the user cadence based on the peak frequency and the frequency threshold comparison(s).

BACKGROUND

Personal health monitors provide users with the ability to monitor theiroverall health and fitness by enabling the user to monitor heart rate orother physiological information during exercise, athletic training,rest, daily life activities, physical therapy, etc. Such devices arebecoming increasingly popular as they become smaller and more portable.

In addition to providing bodily performance information such as heartrate and breathing rate, a personal health monitor may also provideperformance information about the current activity, e.g., duration,distance, cadence, etc. As with many parameters, however, the accuratedetermination of such information may be compromised by noise.

A user's cadence enables the user to monitor their current performancerelative to their personal goals, and therefore represents aparticularly useful piece of activity performance information. As usedherein, a cadence represents the number of repetitions per minute. Forexample, when the user is moving on foot, the cadence represents thenumber of foot repetitions or steps per minute. When the user is movingon wheels, the cadence represents the number of cycle repetitions (e.g.,crank or pedal revolutions) per minute.

Conventional devices may monitor the cycling cadence, for example, usinga cyclocomputer. A sensor system mounted to the crank arm and frame ofthe bicycle counts the number of wheel rotations per minute to determinethe cycling cadence. While such devices are useful and reasonablyaccurate, they are cumbersome and cannot easily be used with multiplebicycles. Further, such devices cannot provide an accurate estimate ofthe number of steps per minute taken, e.g., by a runner. Thus, thereremains a need for more portable devices capable of measuring a usercadence in a wide variety of scenarios.

SUMMARY

The method and apparatus disclosed herein determine a user cadence fromthe output of an inertial sensor mounted to or proximate the user'sbody, e.g., disposed in an ear bud worn by the user. In general, acadence measurement system determines the cadence based on frequencymeasurements acquired from an inertial signal output by the inertialsensor.

An exemplary method determines a user cadence from an inertial signalgenerated by an inertial sensor, where the inertial signal comprises oneor more frequency components. The method determines a peak frequency ofthe inertial signal, where the peak frequency corresponds to thefrequency component of the inertial signal having the largest amplitude.After applying the peak frequency to one or more frequency thresholdcomparisons, the user cadence is determined based on the peak frequencyand the one or more frequency threshold comparisons.

In one embodiment, a cadence measurement system determines the usercadence. The cadence measurement system comprises an inertial sensor anda cadence circuit. The inertial sensor is configured to output aninertial signal comprising one or more frequency components. The cadencecircuit is operatively connected to the inertial sensor, and comprises apeak frequency circuit, a comparison circuit, and a cadence processorcircuit. The peak frequency circuit is configured to determine a peakfrequency of the inertial signal, where the peak frequency correspondsto the frequency component of the inertial signal having the largestamplitude. The comparison circuit is configured to apply the peakfrequency to one or more frequency threshold comparisons. The cadenceprocessor circuit is configured to determine the user cadence based onthe peak frequency and the one or more frequency threshold comparisons.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an exemplary cadence measurement system disposed in an earbud.

FIG. 2 shows a block diagram of an exemplary cadence measurement system.

FIG. 3 shows an exemplary process for determining the cadence from dataprovided by an inertial sensor.

FIG. 4 shows a block diagram of an exemplary cadence circuit.

FIG. 5 shows a more detailed process for determining a user cadence fromdata provided by an inertial sensor according to one exemplaryembodiment.

FIGS. 6A and 6B show simulated results associated with the disclosedsolution.

DETAILED DESCRIPTION

The cadence measurement technique disclosed herein provides an accuratemeasurement of a user's cadence based on a signal provided by aninertial sensor disposed proximate a user's body. FIG. 1 shows part ofan exemplary cadence measurement system 10, where one or more sensors 14are disposed in an ear bud 12, and a cadence circuit 100 is operativelyconnected to the sensor(s) 14, e.g., via a wired or wireless connection.The cadence circuit 100 may be secured to the user, e.g., via a clip.The ear bud 12 may comprise a wireless or wired ear bud thatcommunicatively couples to a remote device, e.g., a music player, asmart phone, a personal data assistant, etc. While not required, it willbe appreciated that the cadence circuit 100 may be disposed in theremote device. While FIG. 1 shows the sensor(s) 14 as being part of anear bud 12, it will be appreciated that the sensor(s) 14 may be disposedin any device that secures to the body of a user, e.g., a device thatsecures to an ear, finger, toe, limb, ankle, wrist, nose, etc. In someembodiments, the device may comprise a patch, e.g., a bandage, designedto attach the system 10 to any desired location on the user's body.While FIG. 1 shows the cadence circuit 100 as being separate from theear bud 12, it will be appreciated that the cadence circuit 100 may bedisposed in the ear bud 12

The cadence measurement system 10 measures the user's cadence, andoutputs the cadence to the user and/or to other processing functions orelements. As used herein, the “cadence” refers to the number ofrepetitions or complete cycles per minute. Exemplary user cadencesinclude, but are not limited to, a step rate (e.g., the number of stepsor foot repetitions per minute), a cycle rate (e.g., the number ofpedaling cycles or cycle revolutions per minute), a repetition rate(e.g., with respect to lifting weights), etc. It will be appreciatedthat a step rate cadence may represent the user's cadence while walking,running, doing aerobics, climbing stairs, etc. Further, it will beappreciated that the cadence measurement system may be used with anymobile animals having one or more limbs that facilitate and/or enablethe animal's movement, or with machinery, e.g., a walking robot.Exemplary animals include, but are not limited to, biped animals (e.g.,humans, birds, etc.) and quadruped animals (e.g., dogs, horses, etc.).

FIG. 2 shows a block diagram of an exemplary cadence measurement system10 according to one exemplary embodiment. System 10 comprises thecadence circuit 100 coupled to one or more sensors 14 and aninput/output interface 16, where the sensor(s) 14 include at least oneinertial sensor 14 a, and an optional physiological sensor 14 b. It willbe appreciated that the inertial sensor 14 a may incorporate aphysiological sensor or physiological sensor capabilities. Inertialsensor 14 a is configured to sense energy, e.g., motion, external to thesystem 10, and to output an inertial signal S_(i) representative of thesensed energy. The inertial sensor 14 a may comprise a single axissensor or a multiple axis sensor. Exemplary inertial sensors 14 ainclude but are not limited to accelerometers, Micro-Electro-MechanicalSystem (MEMS) devices, gyroscopes, optical sensors, an opto-mechanicalsensor, a blocked channel sensor, a capacitive sensor, and a piezosensor. When the inertial sensor 14 a comprises a multiple axis sensor,frequency and power information from each axis may be combined orotherwise evaluated to determine the desired information, e.g., the peakfrequency and the inertial power. For example, the spectral magnitudemay be determined for each axis, where a maximum one of the spectralmagnitudes, a sum of the squares, a maximum of the squares, a sum of theabsolute values, a maximum of the absolute values, the root-sum-squares,the root-mean-squares, and/or the decimation of the spectral magnitudesis ultimately used to determine the inertial power and to identify thepeak frequency. Cadence circuit 100 processes the inertial signal S_(i)as disclosed herein to determine a user cadence C. Input/outputinterface 16 provides input from the user to the cadence circuit, andoutputs the determined cadence C. It will be appreciated thatinput/output interface 16 may include a display, a keyboard or otherdata entry device, and/or a transceiver for transmitting the cadence toa remote device. Alternatively or additionally, the input/outputinterface 16 may provide the cadence to the display, a database, aprocessor, and/or a processing function. FIG. 3 shows an exemplarymethod 200 that may be implemented by the cadence measurement system 10to determine a user cadence C. After the cadence circuit 100 receivesthe inertial signal S_(i) from the inertial sensor 14 a (block 210), thecadence circuit 100 determines a peak frequency f_(p) based on S_(i)(block 220). The peak frequency f_(p) represents the frequency componentof the inertial signal S_(i) having the largest amplitude. The cadencecircuit 100 subsequently applies the peak frequency f_(p) to one or morefrequency threshold comparisons (block 230). Based on the frequencythreshold comparisons, the cadence circuit 100 determines the usercadence C (block 240).

FIG. 4 shows a block diagram of an exemplary cadence circuit 100configured to determine the user cadence from the inertial signal outputby the inertial sensor 14 a. Cadence circuit 100 comprises a peakfrequency circuit 110, a frequency comparison circuit 120, and a cadenceprocessor circuit 130. Peak frequency circuit 110 determines the peakfrequency of the input inertial signal. The frequency comparison circuit120 applies the peak frequency to one or more frequency thresholdcomparisons. The cadence processor circuit 130 determines the usercadence based on the peak frequency and the one or more frequencythreshold comparisons.

The peak frequency circuit 110 identifies the frequency component of theinertial signal having the largest signal amplitude. In one exemplaryembodiment, Peak frequency circuit 110 may achieve this goal byperforming a frequency transform of the inertial signal to determine aspectral signal. The peak frequency circuit 110 then identifies thefrequency component of the spectral signal having the largest amplitudeas the peak frequency. It will be appreciated that other means, e.g.,phase-locked loop, pulse picking, or time-domain implementations, may beused to determine the peak frequency.

The frequency comparison circuit 120 applies the peak frequency to oneor more frequency threshold comparisons. The frequency peak oftencorresponds directly to the user cadence. However, in some instances,the user cadence is some harmonic factor of the peak frequency.Empirical research shows the peak frequency is often twice, half, orthree-halves the user cadence. As shown in FIG. 5, when sprinting ispossible the typical walking harmonics are 2 f_(p), 3/2f_(p), or ½f_(p)and the typical running harmonics are ½f_(p). For example, harmonics at2f_(p) and 3/2f_(p) often occur when the user walks, but not when theuser runs. Thus, the cadence may actually be ½f_(p) or ⅔f_(p),respectively. When the user runs, harmonics at ½f_(p) often occur. Thus,the cadence in this scenario may actually be 2f_(p), ⅔f_(p), or ½f_(p).Thus, the cadence circuit 100 must determine which harmonic factor, ifany, is applicable to determining the current user cadence.

The frequency threshold comparisons applied by the frequency comparisoncircuit 120 as disclosed herein solve this problem using one or morethreshold comparisons, where the thresholds are determined based on aprevious user cadence, an inertial power of the inertial signal, useractivity parameters, user information, and/or empirical values. It willbe appreciated that different harmonic factors and/or thresholds mayapply depending on whether the user is sprinting, walking, running,ramping up from a low frequency value, cycling, etc. For example,harmonic factors due to arm swing, head bobbing, etc., impact the usercadence differently depending on how the user is moving, e.g., whetherthe user is running or walking. Thus, the cadence circuit 100 mayoptionally comprise a power circuit 140, a power comparison circuit 150,a user input circuit 160, a memory 170, and/or a threshold processorcircuit 180 that determine and/or provide the various harmonic factorsand thresholds necessary to determine the user cadence.

The power circuit 140 is configured to determine the inertial powerP_(i) of the inertial signal. To that end, the power circuit 140 maycompute the inertial power in the time domain, e.g., using the root meansquare, or in the frequency domain, e.g., using the amplitude of aspectral peak. The power comparison circuit compares P_(i) to aninertial power threshold T_(i) to facilitate the determination ofwhether the user is running or walking. User input circuit 160 receivesinput from the user. The user input may be used to determine one or moreuser activity parameters, e.g., whether the user is on foot or onwheels, whether sprinting is possible, etc. Threshold processor circuit180 is configured to determine one or more of the thresholds used by thefrequency comparison circuit 120, including any frequency thresholdsused to determine a running cadence, a walking cadence, a cyclingcadence, etc., and the power threshold used by the power comparisoncircuit 150. Memory 170 stores any predetermined thresholds, one or morepreviously determined cadences C_(p), the various harmonic factors usedby the cadence processor circuit 130, and any other information orsoftware necessary for successful operation of the cadence circuit 100.

FIG. 5 shows an exemplary detailed process 300 executed by the cadencecircuit 100 to determine the user cadence C. As shown by FIG. 5, cadencecircuit 100 determines the user cadence based on the peak frequency andone or more frequency threshold comparisons. In exemplary embodiments,the cadence circuit 100 determines a user activity parameter, anddetermines the user cadence based on the frequency thresholdcomparison(s) and the user activity parameter. For example, the useractivity parameter may identify whether the user is on foot or on wheels(block 302). When on wheels, the frequency comparison circuit 120compares the peak frequency f_(p) to a cycling threshold T_(c), whichmay be fixed or variably determined based on an inertial power of theinertial signal (block 310). When f_(p)<T_(c), the cadence processorcircuit 130 sets the cadence equal to the peak frequency (block 312).Otherwise, the cadence processor circuit 130 generates two or more testcadences, and sets the user cadence equal to the test cadence closest toa previous user cadence (blocks 314-322). For example, the cadenceprocessor circuit 130 may generate three test cadences: C₁=½f_(p),C₂=⅔f_(p), and C₃=f_(p) (block 314), and compare the three test cadencesto a previous user cadence C_(p) (block 316). If C₁ is closer to C_(p)than C₂ or C₃ are, the cadence processor circuit 130 sets the usercadence equal to C₁ (block 318). If C₂ is closer to C_(p) than C₁ or C₃are, the cadence processor circuit 130 sets the user cadence equal to C₂(block 320). If C₃ is closer to C_(p) than C₂ or C₁ are, the cadenceprocessor circuit 130 sets the user cadence equal to C₃ (block 322).While the example of FIG. 5 shows determining and using three specifictest cadences, it will be appreciated that any two or more test cadencesmay be used.

When the user is on foot (block 302), the cadence processor circuit 130sets the user cadence equal to the peak frequency divided by a harmonicfactor, e.g., ½, 1, 3/2, 2, etc. More particularly, the cadenceprocessor circuit 130 determines the user cadence based on frequency andpower comparisons respectively performed by the frequency comparisoncircuit 120 and the power comparison circuit 150 (block 330). Forexample, when the inertial power P_(i) is less than the inertial powerthreshold T_(i) and f_(p)≥T_(foot), cadence processor circuit 130generates two or more test cadences based on f_(p) and two or more ofthe harmonic factors, and determines the user cadence based on the testcadences and a previous user cadence (blocks 360-368). For example, thecadence processor circuit 130 may generate three test cadences:C₁=½f_(p), C₂=⅔f_(p), and C₃=f_(p) (block 360), and compare the threetest cadences to a previous user cadence C_(p) (block 362). If C₁ iscloser to C_(p) than C₂ or C₃ are, the cadence processor circuit 130sets the user cadence equal to C₁ (block 364). If C₂ is closer to C_(p)than C₁ or C₃ are, the cadence processor circuit 130 sets the usercadence equal to C₂ (block 366). If C₃ is closer to C_(p) than C₂ or C₁are, the cadence processor circuit 130 sets the user cadence equal to C₃(block 368). While the example of FIG. 5 shows determining and usingthree specific test cadences, it will be appreciated that any two ormore test cadences may be used.

However, when the P_(i)≥T_(i) and/or f_(p)<T_(foot), the cadenceprocessor circuit 130 determines the user cadence based on frequencythreshold comparison(s) and a sprinting user activity parameter, whichindicates whether sprinting conditions are possible (blocks 332-356).More particularly, when P_(i)≥T_(i) and/or f_(p)<T_(foot), the cadenceprocessor circuit 130 determines whether sprinting conditions arepossible based on user input (block 332). For example, the user mayselect an activity mode, e.g., walking, slow or low impact aerobics,high impact aerobics, running, etc. from a menu of options. Based on theselected activity mode, the cadence processor circuit 130 determineswhether sprinting conditions are possible. For example, when the userselects slow aerobics, the cadence processor circuit 130 determines thatsprinting is not possible. Alternatively, when the user selects running,the cadence processor circuit 130 determines that sprinting is possible.If sprinting conditions are possible, the cadence processor circuit 130determines the user cadence based on a comparison between f_(p) and alow frequency threshold T_(low) under sprinting conditions (blocks334-338). When f_(p)<T_(low), the cadence processor circuit 130 sets theuser cadence equal to the peak frequency divided by the ½ harmonicfactor, e.g., equal to twice the peak frequency (block 336). Otherwise,the cadence processor circuit 130 sets the user cadence equal to thepeak frequency (block 338).

If sprinting conditions are not possible, the cadence processor circuit130 determines the user cadence based on multiple frequency thresholdcomparisons under non-sprinting conditions (blocks 340-356). Moreparticularly, the cadence processor circuit applies the peak frequencyto multiple thresholds based on whether the peak frequency is ramping upfrom a low frequency value (block 340), and determines the user cadencebased on that ramping information and the frequency threshold conditions(blocks 342-356). While not required, in some exemplary embodiments, thelow frequency value is zero. During non-sprinting conditions when thepeak frequency is ramping up from a low frequency value, the cadenceprocessor circuit 130 sets the user cadence equal to the peak frequency(block 342).

However, during non-sprinting conditions when the peak frequency is notramping up from a low frequency value, the cadence processor circuit 130determines the user cadence based on multiple peak frequency thresholdcomparisons determined by the frequency comparison circuit 120 undernon-sprinting conditions relative to a low frequency threshold T_(low),an intermediate frequency threshold T_(med), and a high frequencythreshold T_(high), where T_(low)<T_(med)<T_(high) (blocks 344-356).More particularly, under these conditions when f_(p)<T_(low) (block344), the cadence processor circuit 130 sets the user cadence equal tothe peak frequency divided by the ½ harmonic factor, e.g., equal totwice the peak frequency (block 346). When f_(p)≥T_(low) andf_(p)>T_(high) (blocks 344 and 348), the cadence processor circuit 130sets the user cadence equal to the peak frequency divided by the 2harmonic factor, e.g., equal to half the peak frequency (block 350).When f_(p)≥T_(low) and f_(p)≤T_(high) and f_(p)>T_(med) (blocks 344,348, and 352), the cadence processor circuit 130 sets the user cadenceequal to the peak frequency divided by the 3/2 harmonic factor, e.g.,equal to two-thirds the peak frequency (block 354). Otherwise, thecadence processor circuit 130 sets the user cadence equal to the peakfrequency (block 356).

As discussed herein, the cadence circuit 100 determines the user cadencebased on one or more frequency threshold comparisons. Each frequencythreshold, as well as the inertial power threshold, may be determinedempirically or based on one or more parameters, e.g., a previous usercadence, an inertial power, user information, and/or a user activityparameter. For example, the cycling threshold T_(c) and/or the footthreshold T_(foot) may be determined empirically based on observation,and/or based on user input information, user activity parameter, and/orthe inertial power. In one exemplary embodiment, for example, the footthreshold may be determined according to:

$\begin{matrix}{T_{foot} = {120 + {40{\frac{P_{i}}{T_{i}}.}}}} & (1)\end{matrix}$

An exemplary cycling threshold T_(c) is 100 revolutions per minute,while an exemplary foot threshold T_(foot) is 145 steps per minute. Theinertial power threshold and/or the low threshold may be determinedempirically and/or based on user information, e.g., the user's weight,shoe sole compliance information, etc., the inertial power, a previoususer cadence, and/or user activity parameters. In one exemplaryembodiment, T_(low)=60 (a constant). It has been shown, for example,that the low frequency threshold is more accurate when determined as afunction of the inertial power. For example, when P_(i)≤T_(i), the lowthreshold may be determined based on the inertial power when accordingto:

$\begin{matrix}{T_{low} = {60 + {20{\frac{P_{i}}{T_{i}}.}}}} & (2)\end{matrix}$When P_(i)>T_(i), alternatively, T_(low) may be set equal to 80. Inanother exemplary embodiment, the low threshold may be determined basedon the previous user cadence according to:T _(low)=0.6C _(p).  (3)

It will be appreciated that different values for T_(low) may be used fordifferent scenarios. Thus, a combination of the above-disclosed optionsmay be selectively used depending on the different scenarios, e.g.,whether P_(i)>T_(i). Similarly, the intermediate and high thresholds maybe determined based on a previous user cadence and/or the inertialpower. For example, the intermediate and high thresholds may bedetermined as a function of the previous user cadence and a sprintfactor. The sprint factor for the intermediate threshold may bedetermined empirically, e.g., based on 1.75 times the previous usercadence. The sprint factor for the intermediate threshold may also bedetermined empirically, e.g., based on 1.4 times the previous usercadence. It will be appreciated that each threshold may be fixed orvariable. It will also be appreciated that the frequency thresholds(e.g., T_(c), T_(foot), T_(low), T_(med), T_(high)) and the inertialpower threshold (T_(i)) discussed herein are exemplary and non-limiting;other thresholds may be used depending on the system configuration, theinformation available to the cadence circuit 100, etc.

The user cadence method and apparatus disclosed herein accuratelydetermines a user cadence for a wide range of circumstances andenvironments. Further, because the user may wear the hardware necessaryto implement this invention, the invention disclosed herein isapplicable for any user activity, including cycling, walking, running,athletic training, sports, aerobics, weight lifting or any otherrepetitive exercises, jumping, etc.

FIGS. 6A and 6B show simulated results for one exemplary implementationof the cadence measurement system disclosed herein. The plots shown inFIGS. 6A and 6B are generated from the same data set produced by anindividual running and walking on a treadmill. FIG. 6A shows the usercadence with respect to time as computed according to FIG. 5 using thespectral peak frequency provided by the inertial sensor. FIG. 6B showsthe inertial power output by the inertial sensor with respect to time.FIG. 6B also shows an exemplary inertial power threshold of 2000, whichis used to determine whether the user is running or walking/resting. Thecircuits for the y-axis circuits Figure B are “g's” scaled by asystematic multiplier, where 1 g is the force of gravity on Earth at sealevel. As shown by FIGS. 6A and 6B, the user is running from 125-215,seconds and from 300-375 seconds. Thus, in these regions, user cadencemethod and apparatus disclosed herein avoids mistaking the peakfrequencies above 145 steps per minute as 2× or 3/2× harmonics. The40-70 seconds region shows 3/2× and ½× harmonics, the 80-120 secondsregion shows 2× and ½× harmonics, and the 125-215 seconds region shows½× harmonics. All of these harmonics, when divided by the correspondingharmonic factor as disclosed herein, produce the correct user cadence.

In some embodiments, the cadence measurement system 10 may also compriseadditional sensors. For example, the cadence measurement system 10 mayinclude additional physiological sensors 14 b, e.g., blood flow(photoplethysmography (PPG)), body temperature, and/or heart ratesensors.

In some embodiments, a noise circuit 18 (not shown) may be included toremove or otherwise attenuate cadence-related motion artifact noise froma physiological signal based on the determined user cadence output bythe cadence circuit 100. For example, the determined cadence frequencymay be selectively removed from the frequency spectrum of the outputs ofone or more of the sensors 14 so that higher-quality sensor outputs areachieved with substantially attenuated motion artifacts.

In some embodiments, the sensor 14 may comprise an opto-mechanicalsensor, comprising at least one optical emitter and one opticaldetector, such that inertial changes and cadence can be detected by thespectral characteristics of the opto-mechanical sensor output. In suchembodiments, the thresholds described herein may be adapted when appliedto the opto-mechanical sensor to account for differences in the sensingmechanism between an opto-mechanical sensor and an accelerometer,thereby generating an accurate measurement of cadence. In someembodiments, the opto-mechanical sensor may also be configured to shinelight at the skin and detect light scattered from or through the skin toproduce an output signal comprising both photoplethysmography componentsand inertial components. In such embodiments, the cadence measurementgenerated by the opto-mechanical sensor can be removed from theopto-mechanical sensor output to provide a second output having acleaner photoplethysmography signal with substantially attenuated motionartifacts from cadence.

The present invention may, of course, be carried out in other ways thanthose specifically set forth herein without departing from essentialcharacteristics of the invention. The present embodiments are to beconsidered in all respects as illustrative and not restrictive, and allchanges coming within the meaning and equivalency range of the appendedclaims are intended to be embraced therein.

What is claimed is:
 1. A method of determining a user cadence from aninertial signal generated by an inertial sensor disposed in a devicesecured to at least some portion of an ear of a user, the inertialsignal comprising one or more frequency components, the methodcomprising: determining, using a peak frequency circuit disposed in thedevice and operatively connected to the inertial sensor, a spectralsignal based on a frequency transform of the inertial signal generatedby the inertial sensor responsive to limb movement of the user;determining, using the peak frequency circuit, a peak frequency of thespectral signal, said peak frequency corresponding to a frequencycomponent of the inertial signal having a largest amplitude;determining, using a cadence processor disposed in the device andoperatively connected to a frequency comparison circuit, at least onefrequency threshold for one or more frequency thresholds based on aprevious user cadence of the user; comparing, using the frequencycomparison circuit disposed in the device and operatively connected tothe peak frequency circuit, the peak frequency to at least one of theone or more frequency thresholds; determining, using the cadenceprocessor, the user cadence based on the peak frequency and the one ormore frequency threshold comparisons, said user cadence representing amovement of one or more limbs of the user; and outputting the determineduser cadence to an output circuit disposed in the device and operativelyconnected to the cadence processor.
 2. The method of claim 1 whereindetermining the user cadence comprises calculating the user cadencebased on the peak frequency and the previous user cadence when the peakfrequency exceeds a frequency threshold from the at least one of the oneor more frequency thresholds.
 3. The method of claim 1 whereindetermining the user cadence comprises setting the user cadence equal tothe peak frequency divided by a harmonic factor when the peak frequencyis less than a frequency threshold from the at least one of the one ormore frequency thresholds.
 4. The method of claim 3 wherein the harmonicfactor comprises one of ½, 1, 3/2, and
 2. 5. The method of claim 1further comprising: determining, using a power circuit disposed in thedevice and operatively connected to the inertial sensor, a power of theinertial signal, said power of the inertial signal encompassing aplurality of the one or more frequency components of the inertialsignal; and comparing, using a power comparison circuit disposed in thedevice and operatively connected to the power circuit and the frequencycomparison circuit, the power to a power threshold; wherein determiningthe user cadence further comprises determining the user cadence based onthe comparison between the power and the power threshold.
 6. The methodof claim 1 further comprising determining, using the cadence processor,whether the peak frequency is ramping up from a first frequency value,wherein determining the user cadence further comprises determining theuser cadence based on whether the peak frequency is ramping up from thefirst frequency value.
 7. The method of claim 1 further comprising:determining, using a power circuit disposed in the device, a power ofthe inertial signal, said power of said inertial signal encompassing aplurality of the one or more frequency components of the inertialsignal; comparing, using a power comparison circuit disposed in thedevice and operatively connected to the power circuit and the frequencycomparison circuit, the power to a power threshold; and whereindetermining the user cadence comprises determining the user cadencebased on the peak frequency and the previous user cadence when the poweris less than the power threshold and the peak frequency exceeds afrequency threshold from the at least one of the one or more frequencythresholds.
 8. The method of claim 1 further comprising: determining,using a power circuit disposed in the device, a power of the inertialsignal, said power of said inertial signal encompassing a plurality ofthe one or more frequency components of the inertial signal; andcomparing, using a power comparison circuit disposed in the device andoperatively connected to the power circuit and the frequency comparisoncircuit, the power to a power threshold; and determining, using thecadence processor, whether sprinting is possible based on user input; ifsprinting is possible, determining the user cadence based on the one ormore frequency threshold comparisons under sprinting conditions when thepower exceeds the power threshold or the peak frequency is less than afrequency threshold from the at least one of the one or more frequencythresholds; and if sprinting is not possible, determining the usercadence based on the one or more frequency threshold comparisons undernon-sprinting conditions when the power exceeds the power threshold orthe peak frequency is less than the frequency threshold from the atleast one of the one or more frequency thresholds.
 9. The method ofclaim 1 further comprising determining, using a threshold processordisposed in the device and operatively coupled to the frequencycomparison circuit, at least one frequency threshold from the one ormore frequency thresholds based on a power of the inertial signal, saidpower of said inertial signal encompassing a plurality of the one ormore frequency components of the inertial signal.
 10. The method ofclaim 1 further comprising determining, using a threshold processordisposed in the device and operatively coupled to the frequencycomparison circuit, at least one frequency threshold from the one ormore frequency thresholds based on an empirical value.
 11. The method ofclaim 1 further comprising determining, using a user input circuitdisposed in the device and operatively connected to the peak frequencycircuit, a user activity parameter based on user input, whereindetermining the user cadence comprises determining the user cadencebased on the user activity parameter and the one or more frequencythreshold comparisons.
 12. The method of claim 1 wherein the inertialsensor comprises at least one of an accelerometer, aMicro-Electro-Mechanical System (MEMS) device, an optical sensor, agyroscope, an opto-mechanical sensor, a blocked channel sensor, acapacitive sensor, and a piezoelectric sensor.
 13. The method of claim 1further comprising attenuating, using a noise circuit disposed in thedevice and operatively connected to the cadence processor, motionartifacts from a physiological signal proximate in frequency to thedetermined user cadence.
 14. A cadence measurement system disposed in adevice configured to secure to at least some portion of an ear of a userand configured to determine a user cadence, the cadence measurementsystem comprising: an inertial sensor disposed in the device andconfigured to output an inertial signal responsive to limb movement ofthe user, said inertial signal comprising one or more frequencycomponents; a cadence circuit disposed in the device and operativelyconnected to the inertial sensor, said cadence circuit comprising: apeak frequency circuit operatively connected to the inertial sensor andconfigured to determine: a spectral signal based on a frequencytransform of the inertial signal; and a peak frequency of the spectralsignal, said peak frequency corresponding to a frequency component ofthe inertial signal having a largest amplitude; a cadence processoroperatively coupled to the peak frequency circuit and configured todetermine at least one frequency threshold for one or more frequencythresholds based on a previous user cadence of the user; a frequencycomparison circuit operatively coupled to the peak frequency circuit andthe cadence processor, the frequency comparison circuit configured tocompare the peak frequency to at least one of the one or more frequencythresholds; and wherein the cadence processor is further configured todetermine the user cadence based on the peak frequency and the one ormore frequency threshold comparisons, said user cadence representing amovement of one or more limbs of the user; and an output circuitdisposed in the device and operatively connected to the cadence circuit,the output circuit configured to output the determined user cadence. 15.The cadence measurement system of claim 14 wherein the cadence circuitfurther comprises a memory operative to store the previous user cadence,wherein the cadence processor determines the user cadence by calculatingthe user cadence based on the peak frequency and the previous usercadence when the peak frequency exceeds a frequency threshold from theone or more frequency thresholds.
 16. A method of determining a usercadence from an inertial signal generated by an inertial sensor disposedin a device secured to at least some portion of an ear of a user, theinertial signal comprising one or more frequency components, the methodcomprising: determining, using a peak frequency circuit disposed in thedevice and operatively connected to the inertial sensor, a spectralsignal based on a frequency transform of the inertial signal generatedby the inertial sensor responsive to limb movement of the user;determining, using the peak frequency circuit, a peak frequency of thespectral signal, said peak frequency corresponding to a frequencycomponent of the inertial signal having a largest amplitude; comparing,using a frequency comparison circuit disposed in the device andoperatively connected to the peak frequency circuit, the peak frequencyto one or more frequency thresholds; determining, using a power circuitdisposed in the device and operatively connected to the inertial sensor,a power of the inertial signal, said power of said inertial signalencompassing a plurality of the one or more frequency components of theinertial signal; comparing, using a power comparison circuit disposed inthe device and operatively connected to the power circuit and thefrequency comparison circuit, the power to a power threshold;determining, using a cadence processor disposed in the device andoperatively connected to the frequency comparison circuit, the usercadence based on the peak frequency, the one or more frequency thresholdcomparisons, and the comparison between the power and the powerthreshold, said user cadence representing a movement of one or morelimbs of the user; and outputting the determined user cadence to anoutput circuit disposed in the device and operatively connected to thecadence processor.
 17. The method of claim 16 further comprisingdetermining, using a user input circuit disposed in the device andoperatively connected to the peak frequency circuit, a user activityparameter based on user input, wherein determining the user cadencecomprises determining the user cadence based on the user activityparameter, the peak frequency, the one or more frequency thresholdcomparisons, and the comparison between the power and the powerthreshold.
 18. A cadence measurement system disposed in a deviceconfigured to secure at least some portion of an ear of a user andconfigured to determine a user cadence, the cadence measurement systemcomprising: an inertial sensor disposed in the device and configured tooutput an inertial signal responsive to limb movement of the user, saidinertial signal comprising one or more frequency components; a cadencecircuit disposed in the device and operatively connected to the inertialsensor, said cadence circuit comprising: a peak frequency circuitoperatively connected to the inertial sensor and configured todetermine: a spectral signal based on a frequency transform of theinertial signal; and a peak frequency of the spectral signal, said peakfrequency corresponding to a frequency component of the inertial signalhaving a largest amplitude; a frequency comparison circuit operativelyconnected to the peak frequency circuit and configured to compare thepeak frequency to one or more frequency thresholds; a power circuitoperatively connected to the inertial sensor and configured to determinea power of the inertial signal, said power of said inertial signalencompassing a plurality of the one or more frequency components of theinertial signal; a power comparison circuit operatively connected to thepower circuit and the frequency comparison circuit, the power comparisoncircuit configured to compare the power to a power threshold; a cadenceprocessor operatively connected to the frequency comparison circuit andconfigured to determine the user cadence based on the peak frequency,the one or more frequency threshold comparisons, and the comparisonbetween the power and the power threshold, said user cadencerepresenting a movement of one or more limbs of the user; and an outputcircuit disposed in the device and operatively connected to the cadencecircuit, the output circuit configured to output the determined usercadence.